It is well known that a magnetic recording channel will not support direct current signals. In playback, the transducer output is proportional to the time derivative of the flux pattern induced in the transducer. There are also bandwidth limitations inherent in these circuits so that the response to an isolated transition in flux may be modeled as a Lorentzian pulse as the derivative of the arc tangent flux transition.
In digital recording, the input signal is invariably a binary (two-level) non-return-to-zero (NRZ) waveform which generally contains a DC component. The recorder must faithfully reproduce this data pattern upon demand. Thus, the idea of filtering the transducer output to restore the input NRZ waveshape seems natural. Restoration of the input waveform in the analog realm requires, also, the restoration of its DC content. Present approaches to DC restoration require some form of a relatively complicated feedback system, such as decision feedback equalization (DFE). To the applicant's knowledge, none of these feedback schemes are used in commercially available magnetic recorders.